Sol gel, and particularly seeded sol gel aluminous abrasives, have demonstrated substantial advantages over other premium abrasives in broad areas of coated and bonded abrasive applications since their introduction some few years ago. Such abrasives are generally made by drying and sintering a hydrated alumina gel which may also contain varying amounts of additives such as MgO or ZrO.sub.2. The dried material is crushed either before or after sintering to obtain irregular blocky shaped polycrystalline abrasive grits in a desired size range. The grits may later be incorporated in a bonded or coated abrasive product such as a grinding wheel or a segment or a belt or disk.
U.S. Pat. No. 4,314,827 to Leitheiser et al. discloses abrasive grits made by such a method in which the sintered grits contain irregular "snowflake" shaped alpha Al.sub.2 O.sub.3 crystals which are on the order of 5 to 10 microns in diameter. The spaces between the arms of a "snowflake" and between adjacent "snowflakes" are occupied by other phases such as a finely crystalline alumina magnesia spinel.
U.S. Pat. No. 4,623,364, which issued on Nov. 18, 1986 assigned to Norton Company, the assignee of this application, discloses a sol gel method for the manufacture of aluminous abrasive grits, and products other than abrasive grits such as coatings, thin films, filaments, rods or small shaped parts, having enhanced properties. In that patent the conversion of the hydrated alumina to alpha alumina is facilitated by the introduction of seed material into the gel or the gel precursor prior to drying. This can be accomplished by either wet vibratory milling of the gel or gel precursor with alpha alumina media, or by the direct addition of very fine seed particles in powder or other form. To make abrasive grits the seeded gel is dried, crushed and fired. The abrasive grits so produced may be used in the manufacture of products such as coated abrasive disks and grinding wheels. Alternatively, to make shaped parts or rods, the material may be formed or molded as by extrusion before firing. In the case of extrusion, the rods formed are later cut or broken into appropriate lengths.
Once the gel has formed, it may be shaped, by any convenient method such as pressing, molding, or extrusion, and then carefully dried to produce an uncracked body of the desired shape. If abrasive material is desired, the gel can be extruded simply spread out to any convenient shape and dried. After drying, the solid body or material can be cut or machined to form a desired shape or crushed or broken by suitable means, such as a hammer or ball mill, to form abrasive particles or grains.
Such seeded sol gel abrasives have a much firmer alpha Al.sub.2 O.sub.3 crystal structure and higher density than the Leitheiser-type unseeded sol gel material. The alpha Al.sub.2 O.sub.3 crystals of the seeded sol gel abrasives are submicron and usually on the order of about 0.4 microns and less, although somewhat coarser structure may result if the seeding is performed in a non-optimal manner or if the firing is at too high a temperature, or for too long a duration.
Other materials such as Fe.sub.2 O.sub.3 can also be used as seeds to facilitate the conversion to alpha Al.sub.2 O.sub.3. As a rule of thumb, such seeding materials should be isostructural with Al.sub.2 O.sub.3 and should have similar (within about 15%) crystal lattice parameters to work well.
Thus the sol-gel abrasives previously taught have needed to be formed into grits of the desired size by comminuting the larger particles and grading the product. However this procedure is very wasteful since it results in the production of a wide range of particle sizes, not all of which may be useful. This a significant amount of the production has to be discarded, recycled or used in a low value application.
It has now been found that sintered sol-gel microcrystalline materials having significantly enhanced properties can be obtained directly in a particular highly advantageous configuration not requiring further comminution before use. The enhanced properties are particularly surprising since it was considered that the properties of sol-gel microcrystalline alumina abrasives derived from the crystal size and purity, particularly the absence of impurities that are present in, for example, bauxite and which result in the formation of vitreous matter at the temperature at which alpha alumina is formed.
In addition the use of a direct formulation technique results in far less production of materials for which no immediate need exists. In fact essentially all the material produced is of the desired specifications and the process is significantly more efficient as a consequence.